Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, known as a wafer. The components are then wired, or interconnected, together to define a specific electric circuit, such as a computer memory.
Interconnecting and completing the millions of microscopic components typically entails forming contact plugs, covering the plugs and components with an insulative layer of silicon dioxide, and then etching narrow, but often deep, holes in the insulative layer to expose portions of the components, or contact plugs underneath. These holes are then filled with another conductive material, or are developed into additional component parts, e.g., storage nodes for memory cells.
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), is one example of an electrical component requiring contact plugs and etched holes for connection to other portions of an integrated circuit. IGFET's are frequently used in both logic and memory chip applications. An IGFET uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel. Plug contacts and contact openings are required in IGFETs to complete the conductance circuit between the source and drain regions.
Current industry demands are pushing toward increased capacity on individual semiconductor chips in order to yield greater functionality. The push for increased circuit density has been realized through an increase in the miniaturization of individual components, the number of surface layers, and in the depth of contact openings between individual surface layers. Unfortunately, while design rules have shrunk, the registration of layers, or alignment of contacts from one surface layer to the next, has not improved at the same aggressive rate. The problem is compounded by the fact that the very deep contact openings include some taper to them which reduces the alignment tolerance even more dramatically.
Thus a continual need exists for creating improved contact structures, including the formation of contact plugs and contact openings, to improve the registration between semiconductor layers.